From Philips computer monitors in the market the feature lightframe™ is known. This feature enables the user to select an area on the screen of a display device in which the brightness should be increased. This is especially advantageous if natural information is displayed in the area. Natural information comprises photos and films which typically have a lower resolution than synthetic information such as text. The perceptual quality of this low resolution information improves considerably by increasing the brightness, while the brightness of the high resolution synthetic information should not be increased to avoid blurring. Usually, the area is a window or a part of a window created by the operating system and/or by an application running on an operating system.
In cathode ray tubes, the increased brightness is created by increasing the beam current in the cathode ray tube locally in the high brightness area.
In liquid crystal displays, the maximum brightness is determined by the light output of the backlight. If the light output of a predetermined area has to be increased, the light output of the backlight has to be increased and the data outside the predetermined area has to be adapted to keep the brightness substantially constant outside the predetermined area.
A typical backlight lamp driver architecture is disclosed in U.S. Pat. No. 6,078,302. A lamp unit is intermittently driven by a lamp driver circuit which is a current source which supplies the optimal drive current to the backlighting unit. The lamp driver circuit supplies the current to the lamp unit via a controlled switch. A drive current adjuster controls the switch to perform a pulse width control of the drive current. The drive current adjuster can be manually adjusted by a user via a brightness control input.
Due to the current control architecture and the lamp intrinsic behaviour it is difficult to achieve a fast response time of the lamp when a change in the amount of light produced is required.